We are testing the hypothesis that Streptococcus mutans is responsible for a measurable amount of decay in humans. Rampant caries individuals, i.e., individuals with greater than or equal to 10 carious teeth, have high levels of Strep. mutans in their plaque. We are making a bacteriological diagnosis of Strep. mutans infection and treating these individuals topically with either 5% kanamycin or 1.23% NaF in a pleasant tasting gel. The subjects are treated twice a day for a one week period on two occasions, i.e., prior to the placement of dental restorations and immediately after the placement of the restorations. Fissure and proximal plaque samples and saliva are cultured before and after each treatment period and at the recall periods for total viable counts, Strep. mutans and Strep. sanguis counts. A placebo group is included. Subjects are randomly assigned to the treatment groups. The clinical scoring of caries and the bacteriological analysis are performed in double blind procedure. An I.N.D. for chlorhexidine gluconate has been obtained from the FDA and will be used in the treatment of decay in children with serious medical problems. We are conducting longitudinal studies on caries free fissures to determine whether S. mutans colonization predisposes to subsequent caries in the fissures. In separate studies, the acquisition of S. mutans by infants and the intraoral spread of S. mutans in adults is being investigated.